Influence of posture on expiratory flows and airway responsiveness to methacholine in asthma

This study looked at the effects of posture on the morning/evening expiratory flows and airway responsiveness to methacholine. Fourteen nonsmoking subjects with stable asthma (eight men, six women) were included in the study. Subjects were randomly allocated to spend 4 h in the supine or seated position on separate days, in the morning from 8 to 12 AM and in the evening from 8 to 12 PM. The FEV1 was measured hourly in the assigned position. Before and after each 4-h period, a methacholine inhalation test was done in the sitting position. In the morning study, baseline FEV1 measurements on the supine and seated days were not different. There was no significant difference between the baseline and postsession FEV1 on both days (baseline and postsession FEV1 percent predicted +/- SEM; seated: 83.6 +/- 2.9, 83.8 +/- 3.3; supine: 85.8 +/- 2.8, 85.4 +/- 3.7; n = 13). delta FEV1 (baseline/postsession) was not different between the two sessions. In the evening study, baseline FEV1 measurements on the supine and seated days were similar. FEV1 decreased after both sessions, although this difference reached statistical significance only in the supine position (baseline and postsession FEV1 percent predicted +/- SEM; seated: 90.0 +/- 4.1, 84.9 +/- 4.1, p = 0.08; supine: 90.7 +/- 3.1, 82.9 +/- 4.5, p = 0.02; n = 8). delta FEV1 (baseline/postsession) was not different between the two evening sessions. In the morning, after the seated position, PC20 methacholine was unchanged (mean PC20 [mg/ml]: beginning = 1.00, end = 1.02) while after the supine position it was slightly reduced from a mean of 0.97 to 0.73 mg/ml. This last reduction was mainly observed in the most hyperresponsive subjects and its magnitude was significantly correlated with baseline PC20 (r = 0.637, p = 0.024). The increase in methacholine response (delta PC20) after the supine session was significantly higher than after the seated session. In the evening study, there was a slight reduction in PC20 after both sessions, but this was only significant after the supine position (mean PC20 baseline and postsession [mg/ml]: seated: 0.63, 0.47, p = 0.08; supine: 0.62, 0.44, p = 0.04). No difference was found between delta PC20 of the two sessions. We conclude that the supine position does not have persistent effects on FEV1, but it may increase airway responsiveness in the most hyperreactive subjects.